there's probably some gcode you can put in the starting gcode script for s3d.

here's the current heating code:
M126 S[fan_speed_pwm] M140 S[bed0_temperature] T0
M134 T0 ; stabilize bed temperature
M104 S[extruder0_temperature] T0
M133 T0 ; stabilize extruder temperature
I guess if you moved the stabilise bed temperature from where it is to below the stabilise extruder temperature, it might heat both bed and extruder at the same time. It looks like the stabilise command is what makes it wait for the bed to get to temp before heating the extruder.
So if you set it up like this:
M126 S[fan_speed_pwm] M140 S[bed0_temperature] T0
M104 S[extruder0_temperature] T0
M133 T0 ; stabilize extruder temperature
M134 T0 ; stabilize bed temperature
Theoretically it should heat both bed and extruders at the same time. And as the extruder heats faster than the bed there shouldn't be any problems getting the head to temp first.
Theoretically :-)

I generally find when i mess with startup gcode - apparently 'unrelated' things go tits-up.

But I suppose it's worth a try :-)
I know my knp is using a standard pc psu - so has more than enough oomph to heat everything at the same time.